Magnetoresistive random access memory

The present disclosure relates generally to an integrated circuit, and more particularly, a Magnetoresistive Random Access Memory (MRAM).

A Magnetoresistive Random Access Memory (MRAM) stores data in magnetic storage, e.g., magnetic tunnel junctions (MTJs). The reading of the MRAM is accomplished by measuring the electrical resistance of the MRAM cell, which changes according to the magnetic field polarities of the MTJ in the MRAM cell. Data is written to the MRAM cells by storing a magnetic field polarity in the MTJ by using an electrical current (a threshold/critical current). The threshold current affects the power consumption of the MRAM.

The accompanying drawings illustrate embodiments of the invention, as well as other information pertinent to the disclosure, in which:

FIGS. 1 to 1C are perspective views of exemplary MRAM cells according to some embodiments; and

FIGS. 2 to 2D are schematic illustrations of exemplary MRAM cell array write circuits according to some embodiments

This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another, or communicate with one another, either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Terms concerning electrical coupling and the like, such as “coupled”, “connected” and “interconnected,” refer to a relationship wherein elements communicate with one another either directly or indirectly through intervening elements unless expressly described otherwise.

FIG. 1 is a perspective view of an exemplary MRAM cell according to some embodiments. The MRAM cell 100 has an MTJ 102 , a top electrode 104 , a bottom electrode 106 , and a (magnetic field) induction line 108 . The MTJ 102 comprises a free layer 110 , an insulator (spacer, or tunnel barrier) 112 , and a fixed layer 114 . The induction line 108 is located to one side of the MTJ 102 . The MRAM cell 100 , memory arrays including the same, and methods of making the same are fully described in co-pending and commonly assigned U.S. patent application Ser. No. 13/183,968 entitled “Magnetoresistive Random Access Memory and Method of Making the Same”, filed on Jul. 15, 2011, the entirety of which is hereby incorporated by reference herein.

The free layer 110 and the fixed layer 114 form two ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulator 112 . The fixed layer 114 has a fixed magnetic polarity, and the free layer 110 has a changeable magnetic polarity to match an external field to store data. Due to a magnetic tunnel effect, the electrical resistance of the MTJ 102 changes based on the orientation of the magnetic fields in the two plates, i.e., the free layer 110 and the fixed layer 114 . The MTJ 102 has lower resistance if the two plates have the same polarity and higher resistance if the two plates are of opposite polarity in some embodiments.

The MTJ 102 can be manufactured with thin film technology, such as magnetron sputter deposition, molecular beam epitaxy, pulsed laser deposition, electron beam physical vapor deposition, or any other suitable methods. The MTJ 102 is shown to have an oval or elliptical shape, and has different shapes in other embodiments.

The free layer 110 comprises magnetic material, e.g., CoFeB, NiFe, with a thickness of about 15-25 angstroms (Å), while the fixed layer 114 comprises magnetic material, e.g., CoFe, CoFeB, with a thickness of about 40-60 Å (thicker than the free layer 110 ) in some embodiments. The insulator 112 comprises MgO, Al 2 O 3 , or any other suitable material.

The top electrode 104 , the bottom electrode 106 , and the induction line 108 can comprise electrically conductive material, such as copper, copper alloy, aluminum, aluminum/silicon/copper alloy, titanium, titanium nitride, tantalum, tantalum nitride, tungsten, polysilicon, metal silicide, any combinations thereof, or any other suitable materials. The top electrode 104 , the bottom electrode 106 , and the induction line 108 are formed by a process including physical vapor deposition (PVD such as PVD by sputtering), chemical vapor deposition (CVD), plating, damascene processes, combinations thereof, or any other suitable processes. The induction line 108 can be also formed from the same layer as the MTJ 102 in some embodiments.

The MRAM cell 100 shows the direction of a current I on the induction line 108 and the induced magnetic field B from the induction line 108 by the current I. The induced magnetic field B is perpendicular with respect to the magnetic field of the MTJ 102 (i.e., the induced magnetic field is in the Z-direction assuming, for example, the top/bottom surfaces of the MTJ lie in the X-Y plane). The induced perpendicular magnetic field contributes to decrease the threshold (switching) current for writing of the MRAM cell 100 , thus reducing power consumption.

Compared to a thermal assisted (TA) or Joule heating writing method, the MRAM cell 100 does not require a long heating time that is needed for those methods. Also, compared to a writing method using a perpendicular magnetic layer structure, the MRAM cell 100 is more economical.

In the embodiment in FIG. 1 , the induction line 108 is positioned beside the top electrode 104 , but the induction line 108 can be located in other places, in order to induce a perpendicular magnetic field at the MTJ 102 , e.g., above or beside the bottom electrode 106 to one side of the MTJ 102 , or above the top electrode 104 to one side of the MTJ 102 , etc. Also, the induction line 108 can be formed using the same layer as any of the top electrode 104 , the MTJ 102 , the bottom electrode 106 , or in any other electrically conductive layer that can induce a perpendicular magnetic field at the MTJ 102 .

In some embodiments, the MRAM cell is a spin transfer torque (STT) MRAM cell.

FIG. 1A illustrates an alternative MRAM cell 100 A. MRAM cell 100 A is identical to MRAM cell 100 only the induction line 108 A is disposed above (i.e., at a higher level) than the MTJ 102 . In embodiments, the induction line 108 A is formed in a different metal layer than the top electrode 104 . Specifically, in the illustrated embodiment, the induction line 108 A is disposed above and to the side of the MTJ 102 (i.e., there is no overlap with the MTJ). It may be difficult from a layout perspective to locate the induction line immediately adjacent to the top (or bottom) electrode or the MTJ. In order to induce the desired magnetic field with respect to the magnetic field of the MTJ 102 , the current through the induction line 108 A can be adjusted to provide the desired magnetic moment of the induced field at the MTJ 102 . In an alternative embodiment, the induction line is similarly located but below the MTJ rather than above.

Assuming an elliptical shape for the MTJ 102 , which defines short (X) and long (Y) axes, then the MTJ magnetic field will be in the direction of the long (Y) axis. The long (Y) axis is also referred to as the “easy” axis since this axis of the junction is the easiest to magnetize. The location of the top electrode is not important, as only the location of the induction line determines the direction of the induced magnetic field. The induced magnetic field B′ is in the long (Y) axis (i.e., parallel to the field of the MTJ (or parallel to the easy axis of the MTJ)) when the induction line 108 A is oriented parallel to the short (X) axis of the MTJ 102 , and in the short (X) axis when the induction line 108 A is oriented parallel to the long (Y) axis of the MTJ 102 . Orienting the magnetic field either parallel or perpendicular to the field MTJ 102 contributes to decrease the threshold (switching) current for writing of the MRAM cell 100 , thus reducing power consumption.

The induced magnetic field is perpendicular when in the X or Z direction. When the induced field is perpendicular, magnetization switching can be greatly accelerated and the switching current density reduced. The induced magnetic field is parallel when in the Y direction. This induced magnetic field gives the MTJ some magnetic energy and reduces the switching current density.

FIG. 2 is a schematic illustration of an embodiment of a write circuit 200 for a MRAM cell array including MRAM cells as described above in connection with, for example, FIGS. 1 and 1A . It should be understood that while the array is shown as including only two rows 202 and four columns 204 of MRAM cells, this is for illustration purposes only. The 2×4 array includes eight MTJs 206 . The first row 202 a of MTJs is associated with a first word line (WL 0 ) 208 a, and the second row 204 a of MTJs is associated with a second word line (WL 1 ) 208 b. Each column 204 of MRAM cells is associated with a respective source line 210 , source line write buffer 211 , bit line 212 , which may correspond to the top electrode of the MRAM cell, and bit line write buffer 213 . Each column of MRAM cells is also associated with a respective induction line 214 coupled to a respective current source 216 and current sink 218 . The top electrode of the MRAM cell connects with the source line, and the bottom electrode of the MRAM cell connects with the bit line, or vice versa.

For purposes of illustrating the operation of the write circuit, assume that the MRAM cell corresponding to the first row 208 a and second column 204 b is selected for write operation. This MRAM cell is illustrated in dashed line. In order to decrease the threshold (switching) current for writing of this MRAM cell, and thus reducing power consumption, current is provided to the induction line 214 b associated with the MRAM cells of the column 204 b. Particularly in instances where the induction line 214 b is not, or cannot, be located immediately adjacent to the MTJ 206 , such as shown above in FIG. 1A , then the induction line 214 from an adjacent column of cells on the other side of the MTJs 206 can also be provided current to induce a magnetic field at selected MTJ 206 . In this example, current is provided also to induction line 214 a as shown by the current line in both induction lines 214 A and 214 B. With both induction lines contributing the desired magnetic field perpendicular to the magnetic field of the selected MTJ 206 , a desired decrease in the threshold (switching) current for writing to the selected MRAM cell can be achieved.

FIG. 2A is a schematic illustration of another embodiment of a write circuit 200 A for a MRAM cell array including MRAM cells as described above in connection with, for example, FIGS. 1 and 1A . The write circuit of FIG. 2A is identical to the write circuit of FIG. 2 except in one regard—the current in the induction line 214 b flows in the opposite direction to the current in the other activated induction line 214 a. The actual directions of the currents—whether in the same or opposite directions—does not affect the moment of the magnetic field at the selected MTJ 206 .

FIG. 2B is a schematic illustration of another embodiment of a write circuit 200 B for a MRAM cell array including MRAM cells as described above in connection with, for example, FIGS. 1 and 1A . The write circuit of FIG. 2B is identical to the write circuit of FIG. 2 except that two columns of MRAM cells share only one induction line 214 . For example, induction line 214 ab serves both columns 204 a and 204 b, and induction line 214 cd serves both columns 204 c and 204 d. When a MRAM cell from either column 204 a or 204 b is selected for write operation, then current is provided to induction line 214 ab. Likewise, when a MRAM cell from either column 204 c or 204 d is selected for write operation, then current is provided to induction line 214 cd.

FIG. 2C is a schematic illustration of another embodiment of a write circuit 200 C for a MRAM cell array including MRAM cells. The write circuit is identical to the write circuit of FIG. 2 except in three regards. First, the array is illustrates as a 2×2 array of MRAM cells rather than a 2×4 array. This is merely for illustrative purposes. Second, the induction lines, labeled with reference 314 in the figure, overly the MTJs 216 . That is, the induction lines 314 are disposed over, i.e., above and partially cover, the MTJ. These induction lines 314 are described in more detail with respect to FIG. 1B discussed below. Third, one current source/current sink pair 216 , 218 provides current for multiple induction lines 314 via selectable switches SW 1 and SW 2 . For example, if the dashed-in MRAM cell is selected (e.g., the MRAM cell at row 1, column 2), then switches SW 1 b and SW 2 b are triggered to connect induction line 314 b to current source 216 and current sink 218 , respectively. This embodiment saves layout area when compared to embodiments where each induction line has its own current source/current sink pair.

Referring now to FIG. 1B , the MRAM cell 100 B is identical to the MRAM cell 100 of FIG. 1 except for the shape and position of the induction line 108 B. In the illustrated embodiment, the induction line 108 B is positioned over the MTJ 102 such that is fully or partially overlaps or covers the top surface of the MTJ 102 . The induction line 108 B may be formed in a different metal layer than the top electrode 104 . The induction line 108 B could also be positioned underneath the MTJ 102 in a similar fashion. Of particular note, the induction line 108 B also includes an arched portion 109 positioned between otherwise substantially regular shaped, straight portions 111 . The portion 109 arches in plane with the straight portions 111 but in a general direction perpendicular to either the long or short axis of the MTJ. Assuming the straight portions 111 are aligned with the long (Y) axis of the MTJ 102 , then the portion 109 arches out in the X direction, and assuming the straight portions 111 extend along the short (X) axis of the MTJ 102 , then the portion 109 arches out in the Y direction. To the extent the inducting line 108 B overlies multiple MTJs 102 in a single column, it can include multiple arched portions 109 , with a respective arched portion 109 overlying each MTJ 102 . It has been found that using these arched portions 100 can induce significantly more magnetic moment perpendicular to the magnetic field of the MTJ 102 when compared to a straight induction line given the same current. It should be understood that different shapes can provide different intensities for the perpendicular magnetic field. For example, a circle would provide the largest intensity and is an example of another embodiment.

Assuming again an elliptical shape for the MTJ 102 , which defines a short (X) and long (Y) axes, then the induced perpendicular magnetic field B″ is in the Z-direction, regardless of whether the top electrode line 104 is oriented along the long (Y) axis or short (X) axis.

FIG. 1C illustrates another embodiment of a MRAM cell 100 C where the induction line 108 C overlies the MTJ 102 , only in comparison to the induction line 108 B of FIG. 1B the induction line 100 C has a substantially continuous, regular shape without arched portions overlying the MTJ 102 . The induction line 108 C may be formed in a different metal layer than the top electrode 104 . A write circuit 200 D for an array of MRAM cells of the type illustrated in FIG. 1C is shown in FIG. 2D . The write circuit 200 D is identical to the write circuit 200 C of FIG. 1C except for the shape of the induction lines 414 overlying the MTJs 216 .

Assuming again an elliptical shape for the MTJ 102 , which defines a short (X) and long (Y) axes, then the induced perpendicular magnetic field B″′ is in the long (Y) axis (i.e., parallel to the magnetic field of the MTJ 102 ) when the top electrode line 104 and overlying induction line 108 C are parallel to the short (X) axis of the MTJ 102 , and in the short (X) axis (i.e., perpendicular to the magnetic field of the MTJ 102 ) when the top electrode line 104 and overlying induction line 108 C are parallel to the long (Y) axis of the MTJ 102

It should be understood that as with the embodiments where at least two induction lines above and/or to the side of the MTJ are used to induce a magnetic field at the MTJ, induction lines may also be placed both over and under the MTJ to induce a magnetic field at the MTJ. Likewise, combinations are also contemplated, e.g., combinations of induction lines over, under, above, below and/or at sides of the MTJ.

In some embodiments, a magnetoresistive random access memory (MRAM) cell includes a magnetic tunnel junction (MTJ), a top electrode disposed over the MTJ, a bottom electrode disposed below the MTJ, and an induction line disposed above or below the MTJ. The induction line is configured to induce a magnetic field at the MTJ.

In other embodiments, the MRAM cell includes a magnetic tunnel junction (MTJ) having an oval or elliptical shape having short (X) and long (Y) axes, a top electrode disposed over the MTJ, a bottom electrode disposed below the MTJ, an induction line disposed above or below the MTJ, wherein the induction line is configured to induce a magnetic field at the MTJ.

In some embodiments of an MRAM cell array write circuit, the write circuit includes: a plurality of MRAM cells arranged in an array of rows and columns, each MRAM cell comprising a magnetic tunnel junction (MTJ), a top electrode disposed over the MTJ, a bottom electrode disposed below the MTJ; at least one induction line disposed with respect to each of two columns of MRAM cells, each induction line configured to induce a magnetic field at the MTJs with respect to which it is disposed; and at least one current source for providing current to the at least one induction line.

Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variants and embodiments of the invention that may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.